For luminaires, it is advantageous if light can be generated efficiently which has a color locus on or near the Planckian curve, preferably with a color temperature of between 2000K and 4000K or at a standard color locus pursuant to IEC 60081. In particular, light-emitting diodes (LEDs) can be used for this purpose. One aim consists in achieving a high level of color rendering or color fidelity which is virtually constant within wide ranges.
In particular, phosphor-converted light-emitting diodes can be used in a specified range in the Cx-Cy chromaticity diagram above the Planckian curve. In order to achieve a color locus on the Planckian curve, in addition red light-emitting diodes can be used. This makes it possible to achieve a high color rendering index Ra(8)>90.
Luminaires in accordance with the prior art are associated with the problem that brightnesses and color loci of the light-emitting diodes used drift with a change in temperature. The individual light-emitting diodes are also subject to ageing, with the result that over the course of time the color impression imparted by the luminaire changes. A temperature range of from 20° C. (for example when the luminaire is switched on) to 100° C. in a thermally settled state is conventional for the luminaire.
A luminaire is known which has red light-emitting diodes and in which the brightness of the red light-emitting diodes is measured by means of a sensor. The current through the light-emitting diodes or a pulse with modulation (PWM) is adjusted in such a way that the overall color locus of the luminaire is approximately constant.
One disadvantage here is the fact that the color locus changes as the temperature increases (typically by +0.07 nm/K) owing to the shift in the dominant wavelength of the red LEDs. This results in a shift in the overall color locus by approximately three MacAdam threshold value units (TVU) with respect to the original color locus. To this extent, the change in the color locus as the temperature changes can also be perceived by a user.